A high percentage of mines have a shale roof weak in tensile and shear strengths. Known types of roof bolts and support means, including expansion-shell and resin bolts do not work well in this type of material. Resultant failure and roof falls can be extensive, expensive and dangerous to miners. Attempts have been made to utilize composite or plural bolting systems simultaneously such as, for example, expansion-shell or resin bolts with a truss system to combat the problem but difficulties are still encountered.
The tendency of a mine roof to sag between the ribs of the pillars after the coal has been mined is principally a shearing action. Shear has a tension component and a compression component. These two components must be supported or a mine roof will tend to fall. Some rock, such as limestone and sandstone, usually have sufficient tensile and shear strength to support the roof without additional support. Where shale exists in the overlying strata, however, and which is typically weak in tensile and shear strength, heretofore known and used roof supporting systems are not only tested severely but frequently have failed. It is known that shale constitutes a large percentage, for example up to approximately 50%, of roofs immediately above coal seams.
Known and heretofore used installations have not accommodated the various forces which occur or exist in mine roofs and will not accommodate both the shear within the beam and the compression component.
While the mine roof bolts taught in the above-mentioned U.S. patents have, for the most part, proven to be very satisfactory in use, an area for improvement was noted in one particular form of roof bolt especially.
Basically, the present invention improves the form of roof bolt disclosed in the embodiments shown in FIG. 10 to FIG. 16, inclusive, in each of the above patents. This form of the disclosed invention comprised two members, one of which was constituted by a bar member of generally U-shaped cross-section having longitudinal spaced-apart side edges with a stepped edged configuration defining a plurality of angularly disposed, longitudinally spaced flat cam surfaces. The edges generally are of an undulating or stepped wavilinear configuration, and a coactive solid slide member was formed to a generally serpentine shape having an inner surface or face of a stepped configuration comparable generally to the stepped wavilinear configuration or undulation of the edges of the U-shaped member.
In use, the two members in a stacked or nested configuration were inserted into a pre-drilled hole or opening in the roof strata and then axially or linearly displaced, one with respect to the other, into an expanded position wherein the opposing outer surfaces of the members were displaced laterally into pressure-applying relation with the side wall of the opening in the roof strata.
The prior patents not only disclose this type of structure but additionally teach a method of manufacture of the same. It has been found in actual practice that this configuration is not only less expensive to manufacture; it is comparatively light to facilitate handling in the mines; utilizes less material and additionally has proven highly satisfactory when installed.
This prior type of construction in use was capable of easy and quick installation and easily removable after a period of time for reuse. These roof bolts firmly gripped the strata of the roof essentially throughout their length and effectively eliminated anchor slippage and minimized harmful effects of bolt elongation. Roof bolts of this type, in use, pre-stressed the strata or layers of materials above the roof laterally and the prior patents referred generally to a use pattern or arrangement to support compression and tension components of shear stress in the roof layers.
The present invention while directed to a similar construction as described above and as disclosed in the prior patents, incorporates improvements thereover not only in construction, but in individual functional operation of separate roof bolts, and a system for pattern of bolt use resulting in improved roof support.